Phase modulation receiver



May 6, 1947. M, G, CROSBY i 2,420,230

PHASE MODULATION RECEIVER Original Filed Oct. 25, 1941 2 Sheets-Sheet l 64M/wia frirr/an/ |NvENToR M/my afax ATTORN EY May 6,-1947- M. G. CROSBY 2,420,230

PHASE MODULATION RECEIVER Original Filed Oct. 25, 1941 2 Sheets-Sheet' 2 A vv W. f d 4 m, Q @w w mm www A W) M m Patented May 6, 1947 PHASE MDULATHON RECEIVER- Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application October 25, 1941, Serial No.

Divided and this application October 31, 1944, Serial No. 561,289

(Cl. Z50-Z7) 8 Claims.

My present invention relates generally to angular velocity-modulated carrier wave receivers, and is a division of my application Serial No. 416,443, filed October 25, 1941, Patent No. 2,363,- 649. dated November 28. 1944.

Undesired amplitude modulation effects existing on an angular velocity-modulated carrier wave have been removed in the past by means of a limiter, a balanced detector or by a combination of both devices. By angular velocity-modulated carrier wave is meant a phase or frequency modulated carrier wave. The expression timing-modulated may, also, be used hereinafter generically to designate such types of modulation.

The limiter is effective in removing amplitude modulation at all times even in the presence ol frequency modulation, but the balanced detector is only completely balanced for the condition when no frequency modulation is present on the received carrier wave. Hence, when the balanced detector is depended upon to remove the unde sired amplitude modulation the elimination is only complete in the absence of the desired modulation, and becomes progressively poorer as the desired modulation is increased.

Basically, my present invention involves the utilization of detected amplitude modulation existing on the angular velocity-modulated carrier wave to modulate the detected modulated carrier wave energy such phase that the undesired amplitude modulation is greatly reduced. Hence, if desired, the limiter may be dispensed with, since the elimination of the amplitude modulation by the balanced detector is made more complete. By omitting the limiter the gain of the receiver may be decreased, since the large signal voltages required to saturate the limiter do not have to be provided. Economy of construction and simplication follow as a result of gain reduction prior to the demodulator.

Other features of my invention will best be understood by reference to the following descrip tion taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eiect.

Fig. 1A shows the wave form of the detected output of a frequency modulated carrier wave (FM):

Fig. 1B illustrates the same wave form when using the present invention; and

Figs. 2 and 3 show respectively different embodiments of the invention, it being noted that present Fig. 3 corresponds to Fig. 6 of my aforesaid patent. Y

Referring to the accompanying drawings, wherein like reference characters in the dierent `figures designate similar circuit elements, there will first be considered the manner in which the present inverse modulation method effects substantial suppression of the amplitude modulation (AM) existing on the carrier. Fig, 1A

shows the manner in which the presence of amplitude modulation is manifested in the frequency modulation receiver demodulator output. Wave A is the desired detected frequency modulation output which is assumed (in order to' clarify the explanation) to have a higher frequency than the undesired amplitude modulation frequency. The undesired amplitude modulation appears as an amplitude envelope as shown by wave B of Fig. 1A. The undesired amplitude modulation is assumed to produce a 25 percent modulation of the incoming signal.

In the receiver of this invention, an amplitude modulation output is provided having the wave form of the amplitude envelope B. This output is used to Vmodulate the amplitude of wave A in a sense which causes a reduction of amplitude when the wave is at point a on the envelope, and an increase in amplitude when the wave is at point b. The inverse modulation thus applied, therefore, opposes the undesired amplitude modulation which is present, and tends to cancel it. Fig. 1B shows the result of applying the inverse amplitude modulation to the FM wave of Fig. 1A. The resulting AM envelope B is of double frequency and greatly reduced amplitude.

In Fig. 2' there is shown only so much of an FM receiver as is essential to a proper understanding of the invention. Let it be assumed that the balanced detectors E-S are preceded by `the usual pre-demodulator stages of a superheterodyne receiver adapted to operate in the ultra-high frequency band, say 42 to 50 megacycles (mc.) in which FM reception is had. As is well known, the FM wave is produced by devia-ting the carrier frequency in accordance with the modulation amplitude, the rate of deviation being a function of the modulation frequencies perse.

A phase modulated Wave (PM) maybe considered as an FM wave which has been modulated with an input proportional to the modulating frequency. At present, a maximum center frequency (Fc) deviation of kilocycles (kc.) is used for FM broadcasting. In the superheterodyne receiver the FM waves are reduced to an intermediate frequency (I. F.) although the same frequency deviation range is retained. The FM waves, with center frequency of the I. F. value, are to be understood as applied to the resonant primary circuit I tuned to the I, F. value. This invention is applicable to FlVI waves having a high or low frequency deviation ratio.

Diodes 2 and 3 are coupled tothe input circuit I by a network which has been disclosed in my U. S. Patent No. 2,296,092, granted September 15, 1942. Such a detection network provides balanced detected FM voltage and a separate AM voltage. The anode of diode 2 is connected to its cathode through a series path comprising secondary circuit I', resistor R1 and resistor Rz, the two resistors being shunted by the bypass condenser C1. The anode of diode 3 is connected to its cathode through a series path cornprising the secondary circuit I, resistor R3 and resistor R4, the bypass condenser C2 shunting resistors R3-Ri.

Each of circuits I and I is magnetically coupled to tuned circuit I, but circuits I and I are equally and oppositely mistuned relative to the I. F. Value of circuit I by any predetermined frequency value. As explained in my aforesaid Patent No. 2,296,092 detected FM voltage is developed across resistors Rz and R4 which are arranged in polarity opposition, it beine,r noted that the cathode end of resistor R4 is at ground potential.

The detected AM voltage may be taken off at the anode end of resistor R1. This type of discriminator converts applied FM waves to AM waves with modulation envelopes 180 degrees out of phase. These modulation envelopes are detected by diodes 2 and 3 which make the detected outputs available across resistors Ri-Rg and resistors R3-R4. The manner in which resistors Rai-R2 and resistors Rx-Rd are connected together makes the detected FM voltage available across resistors Rza-R4. The detected AM voltage is taken from across resistors R1 and R4. lt will be noted that these two resistors R1 and R4 are connected so that they both have their anode ends at negative potential. and the combination of the detected AM voltage outputs is in phase. The normal adjustments of the resisters are to make them all equal in order to effect balanced conditions for both. AM and FM reception. Bypass condensers will bypass the I. F. currents fed to the discriminator, but allow the modulation freouencies to pass. There will, then. be derived from the upper end of resistor Re the detected FM voltage which. also, has sunernosed thereon the detected AM eilects which are to be eliminated.

'The cathode end of resistor R2 is connected to ground through a series path comprising condenser Cs and resistor 5. An adjustable tan B' is provided to take off from the resistor B any desired magnitude of volta ge thereacross In the same wav the series path comprising con denser C. and resistor 5 acts as a notentiometer connected between. the anode end of resistor R1 and ground. The tao E' slidable along resistor 5 provides an adiustment for feeding a desired magnitude of detected AM voltage to tube Il. The latter acts as an inverse modulator tube.

Tube I! shows a pentagrid tube including a cathode ll. a plate I2 and ve successive grids 4'. I3. A". M and I5. The second input grid l!" is Vconnected bv tap E to the resistor 6. and has tbe detected. Fll/l voltage annlierl thereto. The iirst input grid 4' is connected by tap 5 to resistor 5, and has the detected AM voltage im pressed thereon. The plate I2 of tube 1I is connected to a source of positive voltage through load resistor E0, the screen grids I3 and I4 on either side of grid li being connected to the same source through a voltage reducing resistor lil. The grid I5 is a suppressor and is at cathode potential.

In a modulator of the type shown at tube 4 the carrier is usually fed to the third grid 4, and appears across a tuned circuit in the plate circuit. The modulation applied to the first grid is normally of a frequency such that the tuned circuit does not pass it. However, in this type of audio modulator a resistor I9 is employed in the plate circuit, and there appears across the resistor in amplified form the detected AM voltage originally applied to grid li. There, also, appears across the resistor I0 the inversely modulated detected FM voltage.

In other words, at tube 4 the detected AM voltage is applied to grid 4 in such a phase relation to the detected FM voltage at grid 4 that the wave form of the resultant inversely modulated detected FM voltage is as shown in Fig. 1B. The inverse modulation causes a rise in the gain of modulator tube 4 when the undesired AM causes a reduction of the incoming signal amplitude, and a decrease in gain when the signal amplitude is modulated upward.

The amplified AM voltage appearing as such across resistor Ill may be removed from the modulator output by means of a balancing system embodying tube The latter, for example, may be a twin triode tube. The common cathode connection of tube I is connected through a resistor 8 to the grounded end of potentiometer resistor 8. The upper end of the latter is connected to the plate end of resistor I0 by a condenser C5. The grid 'l' of one of the triodes of tube 'l is connected by an adjustable tap along resistor 8, while the grid l of the other triode is connected by lead I0 to the adjustable tap 5 which feeds the detected AM voltage to grid Il'. The plates of tube 'I are connected in parallel to a common source of positive direct current voltage through load resistor 9.

As the detected AM voltage is amplified by the modulator tube 4 its phase is reversed. When combined in-phase, as is done by tube l, bucking results. Potentiometer 8 may be adjusted so that the undesired detected AM voltage transmitted through the modulator just cancels that fed to the grid l" of tube l'. The output voltage appearing across resistor 9 is, then, the pure modulated output which consists of an inverse amplitude modulation applied to the detected FM wave form. That is, the output appearing across resistor 9 has the wave worm A'-B of Fig. 1B.

At rst it might be assumed that the undesired amplitude modulation may be completely cancelled by this present modulation process. However, that this is not possible will be realized when the condition of amplitude modulation is considered. It is obvious that when the amplitude modulation modulates the carrier to zero amplitude, it would take an inlinite amount of gain to bring it back to normal amplitude,

On the other hand, .for the upward peaks of amplitude modulation the condition of 100% upward modulation may be compensated for by a reduction in gain of 2:1. Thus, the upward and downward swings of the modulation do not follow a lineal' law which allows. complete @acception by a simple application'oi reverse modulation. The degree of reduction. possibleV by reverse modulation processr has been evaluated my aforesaid Patent ifo. 3GB-,64d and need` not be repeated here.

The` method employed in the present invention may be app-lied to the reception ot phase modulated carrie-r waves (PM). The claims ci this divisional application are directed to. such method and the apparatus employed therein. In phase modulation reception the presence of amplitude modulation produces the same kind ot modulation ot the output as in the. case oi- FM waves.Y Consequently, the` same` type` of circuits maybe used for reducing. the undesiredl component. Referring to Eig, which shows; a receiving system for PM waves, the tube iiiis an I. F. amplifier on which are impressed PM waves whose center frequency has beenk reduced to the I. E. value. .The I. E. transformer iii', Whose primary and secondary circuits are each iixedly resonated to the operating I. value, adjusted to be of the band pass-type.

The coil of the tuned secondary circuit is mid-tapped to ground for the s s y ing neutralizing voltage for the piezo-electric crystal e2. The crystal is included in a crystal filter network, and provides a phase modulation discriminator of the overand under-neutralized type as described in my U. S. Patent 2,294,575. It is not believed necessary to describe the crystal lter network, since my latter patent has a full and complete description thereof. The neutralizing condensers 1Q, 'H are adjusted so that an over-neutralized characteristic is presented to one of the two diodes 43 and fle, while an underneutralized characteristic is provided with respect to the other of the diodes. This produces a balanced phase modulation detector which delivers AFC (automatic frequency control) voltage at the terminal designated AFC. Detected PM voltage is developed across the potentiometer resistor 49. It will he understood that the specic PM demodulator shown is but an illustration of one of many types of PM demodulators which may be used.

Tube 41 is a separate multi-grid detector for amplitude modulation detection. For this detection, filtered carrier is obtained by combining the overand under-neutralized outputs of the crystal lilter by means of condensers Bil and el. This combination produces a neutralized iilter output which. is fed to the input grid Se of limiter tube e8. The construction of the latter type of limiter is disclosed in my U. S. Patent No. 2,276,565. granted March 18, 1942. The output of the limiter tube is fed through condenser 92 to the first input grid B2 of detector tube ll'l, while the second input grid 83 is fed through condenser Si! with the unfiltered PM signal energy from the secondary of transformer d i. This combination of filtered carrier and unfiltered PM sign nal energy is adjusted for amplitude modulation detection by means oi phase sbiiters 943-9! or .Q2- llt Reference is made to my aforesaid U. S. Patent 2.276.565 for a more complete description of the operation of limited tube 4B. The limiter functions to hold the filtered carrier fed to grid B4 substantially constant to avoid distortion to selective fading. By using a limiter ahead of the AM detector il a more linear characteristic results because the detector output is proportional to the product of the carrier amplitude and unltered PM signal energy.

The Proper, phase relation for empli-,buds modulation detection ai detector-411s either zero or 180 degrees., The type 0f AM- deieir shown. is that disclosed; in my U. S. Patent No. 21,063,588 granted- Defeber 8. 1936. Reference is mede thereto for a full disclosure of; the relations eX- isting between the voltages on grids 8.2 and 83 to detect AM waves. The c betectedy All/l voltages per se may be taken from across the output load circuit e5 of detector tube di by. leads t. The detected AM voltage is, also, fed` to the third gridl 89 of modulator tube 45 so that its amplitude modulates the detected PM voltage which is fed to the first grid 3G by potentiometer' 4 9. The potentiometer El across resistor 85 provides over leadv 8S. the AM voltage for grid 89, The tube 45 functions as the inverse modulator tube in this circuit. The modulation process occurring at tube il is adjusted to have the right phase tooppose the undesired AM- envelope which exists on the detected PM voltage envelope at potentiometer e9. The phase may be adjusted by reversing the phase of either the ltered carrier or unfiltered signals fed to the grids of AM detector tube 4,1. The tube l operates, as disclosed in Fig. 2, to balance out the amplification of the detected component in the output circuit of the inverse modulator tube 5.

Tube l, a twin trode, functions to balance out the amplified AM component developed during the inverse modulation process by virtue of the inherent amplifying property of tube 45. Since the detected AM voltage fed over lead 88 is applied to grid 89 of tube #15, there will be developed across plate resistor l the said AM voltage in amplied form. Hence, even though the inverse modulation step tends to minimize the undesired AM envelope of the detected PM Voltage, yet the amplification property of tube d5 introduces an amplified AM component in the modulator tube output. The plate side of resistor lili? is connected through lead lill and potentiometer H32 to control grid |03 of the upper triode section of tube l. The control grid I 0l! of the lower triode section is connected to the potentiometer 8l. The plates 95 and |06 of the two triodes are connected in parallel to the upper end of output resistor lill. .The output leads |08 supply the PM voltage component, substantially free of the undesired AM component since the AM voltages on grids ill3 and Ill act to balance out in the common plate circuit of tube 7.

While I have indicated and described one system for carrying my invention into elect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular circuits shown and described, but that many modications may be made without departing from the scope of my invention.

What I claim is:

1. A method of receiving phase modulated carrier waves which have undesired amplitude modulation thereon, which includes deriving a rst modulation voltage from the waves corresponding to the phase modulation and undesired amplitude modulation, deriving from the phase modulated carrier waves filtered carrier energy, combining said filtered energy and unltered phase modulated carrier Waves in such phases as to provide a second modulation voltage correspending to said undesired amplitude modulation, and subjecting both said modulation voltages to a modulation step in such relative phases that said undesired amplitude modulation of said rst modulation voltage is substantially reduced.

2. In a method as defined in claim l, substantially compensating the amplication eiect of said modulation step on said second modulation voltage by combining said second modulation voltage with the product of said modulation step in opposed relation,

3. A method of deriving the desired modulation from phase modulated carrier Waves which is provided with undesired amplitude modulation, including deriving from said Waves a modulation voltage representative of the phase modulation and undesired amplitude modulation, deriving from said waves a second modulation voltage representative of said undesired amplitude modulation, and inversely modulating the rst modulation voltage with the second modulation voltage thereby to reduce said undesired amplitude modulation.

4. In a method as donned in claim 3, balancing out said second modulation voltage, produced in amplified form during said inverse modulation, by combining in balancing relation said second modulation voltage with the product of said inverse modulation.

5. In a method as deried in claim l, the step of limiting said filtered carrier energy prior to said combining step.

modulating the first modulation signal with the second modulation signal, and additional means for balancing out said second modulation signal amplied during said inverse modulation,

8. A method of receiving carrier Waves Whose modulation includes both phase and amplitude components, separately detecting the Waves to provide a modulation signal having both components, and a second modulation signal having essentially said amplitude component, and combining the two modulation signals in an inverse modulation step to reduce to a substantial extent said amplitude component.

MURRAY G. CROSBY. 

